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Topic: B31 crankshaft/ cushdrive woes (Read 6689 times)

I'm building a B31 from parts, I have a 2 lobe cushdrive but not the retaining nut/collar to screw it to my crankshaft. The problem is I've discovered that threads where it screws on, measure 11/16" on my crankshaft, whereas my pals crankshaft measure 3/4" (Draganfly list this size). Does anybody have any thoughts on why my crankshaft thread is a different size? It has the 66 610 casting stamp which I believe is normal, and it fits into the bearings and cases fine. Is there a smaller size retaining nut/collar?I could look for a complete cushdrive but I'll still have o find a nut for the smaller crank thread whichever cushdrive I use.All suggestions welcome.

It looks as though the 1953+ B31's used cush drive nut 42-71 which has a 3/4 x 20 tpi thread. However, earlier B31's used cush nut 65-2520 but, unfortunately, I can't seem to find what thread this used. It's possible this used a 11/16 thread? It seems most B31 crankshafts were stamped 66-610 but do you know what year the engine is dated or is it just a box of bits?

Hi ArthurIt's a box of bits I'm afraid. Shaft thread seems to be 22 tpi, if I'm reading my gauge correctly. I had thought if all else fails I'd find a nut and weld a big washer to it, but as yet, I can't find any reference to 11/16 22tpi.

As said, the dia and pitch doesn't seem to match any obvious thread form. I assume this is a B31 crankshaft as its part number would suggest - but have you checked the diameter of the journals and it's fit inside the crankcases?

If it was me, I would probably take an existing cush drive nut, re-sleeve it, and then cut the appropriate thread to match that on the crankshaft. This would have to be done using a screw cutting lathe as a suitable tap wouldn't be available.

Thanks one and all for the comments. I've just been out to check my measurements and I can confirm that the shaft thread is 22 tpi and 11/16" and measured my pals which is 20 tpi and 3/4". It beats me as to why mine would be an odd size.I don't have access to the type of equipment to make a suitable nut, and I haven't found a listing for one, so I guess I'll simplest solution would be a replacement crankshaft, which hurts a bit having spent a lot of time truing mine.I could be interested in yours Andy, what were you thinking, and where are you based?

Thanks Andy, I'm a couple of hours away in North Yorkshire so postal costs will need to be considered. I've just bought a new big end so it wouldn't matter if I got a shaft with a knackered big end. Let me check prices and costs.

The B31 used the same drive side mainshaft from 1946 until 1957 when it changed for the alternator engines. Part 65 1128 for the shaft and nut 65 2520 until 1955 when it changed number.

So the thread would be 3/4 inch x 20 tpi cycle for both cush drive nuts.

I dont recognise a 22 tpi thread. A cycle thread would be 20 tpi or 26 tpi.

22 tpi is fairly close to a 1 mm pitch metric? And 18 mm is close to 11/16?

Colbrew, Whitworth did a different size on their threads, some of those were 22TPI, most likely that's what you have, if I remember correctly I had to purchase a tap, I think it was for a military M20 axle. HTH

Thanks Andy, I'm a couple of hours away in North Yorkshire so postal costs will need to be considered. I've just bought a new big end so it wouldn't matter if I got a shaft with a knackered big end. Let me check prices and costs.

Thanks John, if you are correct that would explain the odd size and if I could find any sort of nut with that thread then I'd be able to make a retaining nut and save buying a new crankshaft, which would be a great relief given the time I've spent building this one.I'll start searching for a nut or a tap before splashing out on a replacement shaft.

I've looked high and low to identify the thread but the best I can find is that it might have been an obsolete American Standard thread which were all superseded by Unified threads during WW2. Nowadays, the closest you can get appears to be 11/16-20 or 11/16-24.

In principle, it would be possible to cut the thread on a screw cutting lathe - I think the thread size is just about big enough to get the cutting tool in. However, as Julian says, the issue would then be the correct grade of steel to be used and whether it could be easily machined. SRM do a replacement cush drive nut which is made from hardened steel but I'm not sure what a conventional cush drive nut is made from without seeing one. If it's made from hardened steel then it would be much more difficult to modify it.